This section is intended to provide a background or context to the invention disclosed below. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented or described. Therefore, unless otherwise explicitly indicated herein, what is described in this section is not prior art to the description in this application and is not admitted to be prior art by inclusion in this section. Abbreviations that may be found in the specification and/or the drawing figures are defined below, after the main part of the detailed description section.
Fronthaul is used to carry baseband data from the baseband units (BBU) to the remote radio units (RRU). The Fronthaul function is to enable the baseband units to seamlessly connect to the remote radio units without impacting radio performance. In modern cellular architecture, baseband units and remote radio units are no longer simple one to one relationship. For example, centralized baseband units can support a plural of remote radio units that are geographically separated. However, for example, it is not economical to run a separate fiber to each radio unit. As a result, baseband data for several radio units that are geographically close to each other (radio unit cluster) can be sent together in a single fiber. Along the same lines, one can use a single fiber to carry baseband data for several radio unit clusters to a geographically neutral location (or a HUB), then split the baseband data to different radio unit clusters in separate fiber cables.
Radio over Ethernet has gained a lot of attention recently due to its cost effectiveness and scaling flexibility. Frequency domain antenna data over Ethernet is now considered the preferred inception point for 5G technology to enter into 3GPP standard for Cloud RAN architecture. 3GPP and other standard bodies like IEEE NGFI WG (Next Generation Fronthaul Interface Work Group—represented by various operator and equipment vendors) are also actively pursuing standardization of radio over Ethernet.
In Nokia's Cloud RAN roadmap, one approach is the so called L1′ split, which is transporting frequency domain data over Ethernet, as shown in FIG. 3.
In this architecture, the frequency domain data are transported over the fronthaul to the radio unit (which may or may not be remote), where they are converted to time domain through IFFT, a cyclic prefix is inserted and the signal is sent to the RF block for additional signal processing before going over the air. The fronthaul is the serial connection between baseband controllers and standalone radio heads. The fronthaul length may range, for example, from less than a few meters to 40 or more kilometers. If the radiohead is remote, the baseband controllers are separated from the radio units, but not necessarily geographically far away.
In order to be bandwidth efficient, compression is used on the frequency domain data. One typical compression algorithms is u-law compression. The commonality of compression algorithms is to use a fewer number of bits to represent the original data, which requires a larger number of bits. In order to improve the performance, it is desirable to reduce the dynamic range of the data to be compressed.
However, in the conventional LTE implementation, the dynamic range of the frequency domain signal can vary significantly due to, among others, the following reasons:
(1) For the control channel symbol region of the downlink, the frequency domain power can vary greatly due to power control and non-contiguous resource allocation. In the symbols that carry PDCCH, some REGs (Resource Element Group) can have large values for cell edge users while the neighboring REGs can have much lower value for cell centre users, or certain REGs can be left un-used. The same can be said for PHICH where the dynamic range can be large due to user multiplexing as well as power control.
(2) For the shared user traffic region of the downlink, the dynamic range of the frequency domain power can also be large due to precoding and to a lesser degree, due to higher order modulation (for example, QAM256).
(3) Any pilot boosting on the downlink can affect the dynamic range of the frequency domain data for compression. In addition, in 3GPP 5G preliminary specification (5G_211, 5G_213), a new concept of downlink power boosting is proposed. This power boosting method suggests increasing the downlink power over the resource blocks (RBs) for cell edge users while reducing the downlink power over the resource blocks for near users.
As an example of reducing PAPR, Uplink SC-OFDM for LTE utilizes SC-OFDM in LTE to reduce the PAPR for power saving purposes. But this SC-OFDM is on a per-UE basis, and is only used on portions of PUSCH Resource Blocks.
As an example of frequency domain conversion, methods are known that apply FFT to time domain signals to convert it to frequency domain, and transport the frequency domain data over the fronthaul interface. In this case, FFT is applied to the entire signal.
In summary, it is typically not optimal to do block compression/decompression on the frequency domain data directly due to the large dynamic range in the data block. Accordingly, there is a need to reduce the dynamic range in order to improve the compression/decompression performance.